1. Field of the Invention
This invention relates to an optical system for rotating an image, and more particularly to an optical system using an image rotating prism for rotating an image in a taking lens system for cameras such as television cameras, motion picture cameras and photographic cameras. The image rotating optical system in accordance with this invention is also applicable to various other types of optical instruments.
2. Description of the Prior Art
It has been well known in the art to rotate an image in an image forming optical system using an image rotating prism. The image rotating prism has an odd number of reflecting faces, and the incident light and the exit light thereof have a common optical axis. The image formed by the light passing through the rotating prism is rotated about the optical axis thereof by rotating the prism thereabout.
This kind of image rotating prism is usually located before an objective lens. However, the image rotating prism located before the objective lens cannot sufficiently cover the visual field of the objective lens, since the objective lens usually has a wide visual field and the entrance face of the image rotating prism cannot be made so large in comparison with the effective optical path length of the prism. If the entrance face of the prism be made so large as to cover the visual field of the prism, the whole size of the prism would be impractically large.
The above described drawbacks of the image rotating prism located in front of the objective lens become far more serious when the objective lens is a zoom lens which has a front lens component of large diameter with a variable visual field angle and has the entrance pupil deep in the lens system.
It is obvious that the above described drawbacks of the conventional image rotating prism are based on the fact that the aperture of the objective lens is located outside the prism. The entrance face of the prism, therefore, can be made small if the aperture of the objective lens is brought into the prism. Theoretically, the size of the entrance face of the prism and the optical path length of the prism can be made smallest by locating the aperture of the objective lens at the center of the prism. It is impossible, however, in a practical sense to place the aperture of the objective lens in the optical path of the image rotating prism. Further, the size of the prism inevitably becomes large in order to widen the angle of the visual field of the prism.